The long-term objectives of my laboratory are to improve our understanding of the basic regulatory mechanisms that govern drug disposition and drug metabolism. There is a clear recognition that the frequency of adverse drug reactions (ADRs) is increased in patients affected with an underlying systemic inflammation. In these individuals, variable drug responses are not necessarily related to genetic polymorphisms but are due to the rapid and dramatic reduction in the expression of key genes that encode important drug metabolizing enzymes (DMEs) and drug transporter proteins. In liver, the pregnane x receptor (PXR) is the master- xenobiotic 'sensor'. Ligand-dependent activation of PXR increases the overall uptake, metabolism, transport and eventual elimination of a myriad of xenobiotics and prescription drugs. Based upon strong preliminary data, we hypothesize that the immunosuppressive effects that occur in patients following Rifampicin therapy occurs via the post-translational modification of the PXR protein. We provide evidence that TNFalpha promotes poly-sumoylation of liganded-PXR to feedback repress NF-kappa B-target genes. Aim 1 will define the specific sites of ligand-activated PXR SUMOylation sequentially in vitro, in cultured cells, and then in hepatocytes using a novel and innovative mass spectrometry-based approach. Aim 2 will test the hypothesis that the mechanism of selective repression of the inflammatory response is due to SUMO-modified PXR preventing clearance of nuclear receptor corepressor/NF-kappa B complexes using biochemical methods. Understanding the biochemical details and molecular mechanisms of how PXR is converted from a positive regulator of hepatic DMEs into a transcriptional suppressor of inflammation in liver tissue will provide a new molecular paradigm for understanding drug-mediated repression of the hepatic inflammatory response. New molecular insights regarding the biology of PXR SUMOylation will provide new opportunities to develop novel pharmacological strategies for addressing ADRs, and will eventually help to identify small molecules that will be used to treat inflammatory liver diseases.